Deductive reasoning is seen as having two components. One, the logical component, consists of a logical vocabulary for composing propositions, and a repertory of deductive steps. The other, the performance component, includes two main programs, (1) comprehension processes that determine the information accepted from the premises, and (2) routines and strategies for constructing a line of reasoning. A model of the logical component embodies the notion that the function of if-then is to provide a grammatical frame for stating inference rules; a set of inference rule schemas define the repertory of deductive steps used in propositional reasoning. One goal is to test this model and extend it to reasoning from quantified premises. In the main method, the model, considered in conjunction with plausible performance assumptions, is made to predict the mental steps that people go through in solving problems: differences in errors and/or response times as a function of the kind of problem are predicted and tested. The other goal is to clarify the nature of the improvement in deductive reasoning that takes place from about age ten to adulthood. The developmental changes are expected to lie primarily in the performance component, the inference rule schemas being available at an early age.